This invention relates to x-ray filters for controlling the energy of an x-ray beam and specifically to filters used in x-ray computed tomography systems for making dual energy measurements.
Computed tomography (CT) systems typically include an x-ray source collimated to form a fan beam directed through an object to be imaged and received by an x-ray detector array. The x-ray source, the fan beam and detector array are orientated to lie within the x-y plane of a Cartesian coordinate system, termed the "imaging plane". The x-ray source and detector array may be rotated together on a gantry within the imaging plane, around the imaged object, and hence around the z-axis of the Cartesian coordinate system.
The detector array is comprised of detector elements each of which measures the intensity of transmitted radiation along a ray path projected from the x-ray source to that particular detector element. At each gantry angle a projection is acquired comprised of intensity signals from each of the detector elements. The gantry is then rotated to a new gantry angle and the process is repeated to collect an number of projections along a number of gantry angles to form a tomographic projection set. Each acquired tomographic projection set may be stored in numerical form for later computer processing to reconstruct a cross sectional image according to algorithms known in the art. The reconstructed image may be displayed on a conventional CRT tube or may be converted to a film record by means of a computer controlled camera.
The x-ray source is typically comprised of an evacuated glass x-ray tube containing an anode and a cathode. X-rays are produced when electrons from the cathode are accelerated against a focal spot on the anode by means of a high voltage across the anode and cathode.
The spectrum of the x-rays produced encompasses a band of radiation of different frequencies and hence different energies. The short wavelength radiation of higher energy is referred to as "hard" x-ray radiation and the longer wavelength radiation of lower energy is referred to as "soft" x-ray radiation. The very lowest energy x-rays are almost entirely absorbed by the body and therefore provide little contribution to the x-ray image. Nevertheless, these soft x-rays contribute to the total exposure of the patient to harmful ionizing radiation. Accordingly, these rays are usually removed by a filter incorporated into the x-ray tube, as is known in the art.
The x-rays emitted by the x-ray tube may be subjected to two additional filters, a "spectral" filter and an "attenuation" filter.
The spectral filter may be a molybdenum strip which serves to harden the x-ray beam by further removing longer wavelength, lower energy x-rays. This spectral filter may be moved in and out of the beam of x-rays and hence provides the ability to image an object with x-ray beams of different spectral composition. The construction of x-ray images from two or more images taken with x-ray beams of different spectral composition is termed "dual energy scanning" and finds considerable use in the imaging of soft tissue where single energy scanning may only provide limited contrast. The spectral filter may be equipped with a track or hinge to permit its introduction and removal from the x-ray beam.
In addition to the spectral filter, the x-rays may be subjected to an attenuation filter. The attenuation filter is ordinarily a synthetic polymer such as Teflon having an x-ray absorption spectral characteristic near to that of water and hence the human body. This filter is not intended to adjust the spectral characteristics of the x-ray beam but rather to compensate for the variation in thickness of the imaged body The x-rays that pass through the center of the imaged body, ordinarily the thickest part, are least attenuated by this filter whereas the x-rays passing through the edges of the imaged body, ordinarily the thinnest part are more attenuated by this filter. The x-rays that are not intercepted by the body at all are maximally attenuated by this filter, ideally by an amount equal to that of the x-rays passing through the center of the body. The result of this selective attenuation is that the x-rays striking the Ct x-ray detectors are of similar energy and centered around the middle of the detector's sensitivity. The attenuation filter therefore may allow the use of more sensitive x-ray detectors reducing the range of x-ray energies.
For purposes of calibration it is ordinarily desirable that the attenuation filter may be removed from the path of the x-ray beam. This may be accomplished by positioning the attenuation filter on a movable track.
As mentioned, the attenuation filter may be constructed of a synthetic polymer such as Teflon so as to closely match the absorption characteristics of the imaged body. Although the absorption characteristics of such polymers may be relatively stable, under continued x-ray exposure the mechanical characteristics of the polymer change. The color of the material may darken and cracks may develop. The discontinuity of the x-ray beam introduced by the cracks may cause severe image artifacts and thus require the replacement of the filter. Such replacement may be both inconvenient and costly.